At present, many different methods of communication are known to utilize a multiple access mode. These methods are realized in systems with frequency, time or code division of channels. Such systems are called Frequency Division Multiple Access (FDMA) systems, Time Division Multiple Access (TDMA) systems, and Code Division Multiple Access (CDMA) systems, respectively.
Each of these systems has its own advantages and disadvantages. For example, FDMA systems easily provide a necessary channel by means of tuning the receiving filter at the necessary frequency. However, a reverse side of this advantage is the limitation on additional channels due to the bounded operation bandwidth. TDMA systems allow communications even at the same frequency by means of sequential transmission of information fragments from different users. However, in this case there is also the same limitation as in FDMA systems on the number of the channels due to a finite length of the transmission slot that is repeated with a predetermined period. CDMA systems allow increase in the transmission rate. However, these systems also have the limitation on the number of channels due to the use of pseudo-random sequences (PRS) for individual code forming, because the number of such PRS is finite. As a rule, in these communication systems, the channel division methods are used together.
Furthermore, another problem in those systems is difficulties in forming an asynchronous data sequence since, in actual practice, data from numerous users come to a transmission station of such a system at arbitrary moments. In TDMA systems, a sequential transmission of information fragments is performed. The information fragments come from different users and have the same duration during each time frame in a common sequence. Forming of such a data sequence is rather appropriate in the absence of so-called priority users, e.g., emergency or rescue services, in the communication system, and with the proviso that the communication system has more or less uniform load. However, in practice, the load is rarely uniform and moreover, the operation becomes complicated in the presence of, e.g., priority users or assigned priority channels, when the forming a sequence of data transmitted from multiple users has a clearly defined asynchronous nature. In CDMA systems, information symbols of users are encoded by code words having the same number of bits. Even in those communication systems having a possibility to change a bit number in the code words for adapting to specific conditions in the communication channel, this changing of the bit number occurs simultaneously for all code words. In so doing, it is as difficult to provide an asynchronous transmitted data sequence, as in the case of TDMA systems.
A method for forming a signal system for multiple access communication is disclosed in U.S. Pat. No. 5,570,351 issued on Oct. 29, 1996 to Wornell. In this patent, information sequences for each user are coded by means of a convolution with the so called extended signature of that user, i.e., with a certain random sequence assigned to that user and unknown to any other user. Thus, this method pertains all the disadvantages of the CDMA, including the limited number of users.
Another method for forming a system for multiple access communication is disclosed by Dmitriev A. S. et al. in an article entitled “Dynamic chaos: A paradigm for modern communication systems” (Foreign radioelectronics. Advances in modern radioelectronics, 1997, No. 10, pp. 13–14). The method uses chaotic signals. According to this method, at least one dynamic chaotic system is constructed, such that the system has a strange attractor in its phase space, which is a plurality of chaotic trajectories and includes a set of a countable number of unstable periodic skeletal orbits. The unstable skeletal orbits are determined by the structure of oscillation of the dynamic chaotic system. Chaotic signals corresponding to a plurality of the unstable skeletal orbits are selected according to predetermined rules. Information messages consisting of the alphabet symbols of the users are received from users of the communication system. A common sequence from the symbols being received from users is formed so that a symbol follows an order defined in accordance with a predetermined rule. The dynamic chaotic system is successively tuned to generate the chaotic signals, each of which corresponds to the specific symbol of the formed common sequence. Then, in a communication channel, an asynchronous data sequence is formed from the chaotic signals generated successively. This method uses a spread-spectrum communication system. Advantages of spread-spectrum communication systems due to the use of chaotic signals are the ease of implementation and a possibility of constructing self-synchronizing circuits, stable with respect to various interferences. However, the article discloses only the principle for arranging a communication by chaotic signals, and does not disclose any concrete steps necessary for forming the chaotic signal system or asynchronous data sequences in a multiple access communication system.
In the existing multiple-access communication systems using the transmission with an asynchronous data sequence, an important feature is a possibility of such a system to extract, at the receiving side, those signals which are destined for a specific user.
The above mentioned article also discloses a method for extracting information from an asynchronous data sequence. An alphabet is assigned in advance to every sender-recipient pair in the communication system. Symbols of the alphabet differ from symbols of alphabets of at least some other sender-recipient pairs. At a transmitting side of the communication system, an asynchronous data sequence is formed from chaotic signals which are being sequentially generated by the dynamic chaotic system in accordance with an information message from each user. At a receiving part of the communication system, every recipient is provided with a selecting system tuned at extracting chaotic signals corresponding to symbols of alphabets assigned to this recipient in any sender-recipient pair in which this specific recipient takes part. Upon the reception of the asynchronous data sequence by a specific recipient, the recipient extracts the chaotic signals destined for this recipient from the asynchronous data sequence by the selecting system. However, this article describes only the principle for extracting information from an asynchronous data sequence in a multiple-access communication system formed on the basis of a chaotic signal system, and does not disclose any details concerning the specific steps that are necessary to extract the information from such an asynchronous data sequence.